Increasingly, renewable liquid biofuels are being evaluated on their carbon footprint measured in grams of carbon dioxide equivalent per megajoule of energy content (g CO2e/MJ), which is then compared to the same metric for the fossil fuels (gasoline-diesel fuel), which they are intended to replace.
The United States has invested heavily to build a renewable liquid-fuel industry that is largely based on the production of ethanol from dry-milled corn. This large industry uses around 40% of the U.S. corn crop to produce 15 billion gallons per year of corn-derived ethanol, which is the federal government's target production for 2015.
Nonetheless, there is continuing pressure on this large, well-established industry to continue to lower its carbon footprint. The industry continues to use relatively energy inefficient unit operations; for example, the conventional method of drying distillers' dried grains with solubles (DDGS) is a very inefficient single-pass rotary-drying system. Further, because the drying of DDGS involves the removal of residual ethanol, that ethanol vapor is usually emitted to the atmosphere as a volatile organic compound (VOC), which adversely impacts air quality.
To minimize the risk of high VOC emissions, the U.S. federal Environmental Protection Agency (EPA) has mandated the inclusion of a regenerative thermal oxidizer (RTO) to burn the exhaust from the DDGS dryer to eliminate the risk of VOC emissions. The RTO needs to burn a significant amount of natural gas in the process of destroying the residual ethanol vapor. It would be desirable to avoid the need for a RTO in an integrated system.
Compared to corn wet milling, corn dry milling has historically focused on ethanol production. There are several market drivers to improve corn dry milling and utilize principles of integrated biorefineries, including multiple co-products, efficient recycling, and energy integration. One need is to produce more edible corn oil than conventional processes. There is also a desire to avoid the use of hazardous hydrocarbon-based solvents to produce edible corn oil. Another need is to reduce the amount of residual ethanol in the DDGS co-product.
Most importantly, from the viewpoint of overall sustainability, what is needed is a process of corn dry milling that significantly reduces energy usage and lowers (fuel) ethanol's carbon footprint.